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How to convert to RGBA?


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#1 Acharis   Crossbones+   -  Reputation: 3663

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Posted 01 January 2013 - 06:47 AM

I have SDL_TTF to render text then I convert it to a texture and display. Everything is fine as long as I use TTF_RenderText_Blended(), but with TTF_RenderText_Solid() it's not working, since it's making a non RGBA surface...

So, I need somehow to convert the surface to RGBA surface, so I can use it with OpenGL's GL_RGBA.



I tried SDL_ConvertSurface() but it's not working, most likely due to wrong PixelFormat (I have no clue how to setup a proper RGBA pixelformat, I tried to copy one by one all values (except palette) from the primary surface but it's not working and the code is ugly as well). How to make the conversion?

Edited by Acharis, 05 January 2013 - 09:47 AM.

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#2 darrenc182   Members   -  Reputation: 323

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Posted 01 January 2013 - 09:59 PM

An RGBA image is just an image with 4 channels.  I am going on the assumption that you mean 1 channel not 1 bpp, because 1 bpp(bits per pixel) is impossible in regards to an image as far as I know.  And instead of 4bpp I think you mean 4 channels because again I think 4bpp(bits per pixel) is impossible.  Given this all you have to do is create a 4 channel image array, pack the red, blue and green elements of the array with the colour data from the 1 channel image data and for the 4th channel (the alpha channel), use a value of 255 or 1.0f indicating that all of the colour needs to be visible.  You could always just use a RGBA image from the start but since you did not post any code I do not know what you are doing.  Hopefully this helps even though it is vague, of course with limited information comes a limited explanation.



#3 Acharis   Crossbones+   -  Reputation: 3663

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Posted 02 January 2013 - 03:51 PM

Oh, by BPP I meant BytesPerPixel (from fontSurface->format->BytesPerPixel). It can be indeed confusing.

 

 

Anyway, that's the problem:

fontSurface=TTF_RenderText_Blended(font, string, col);
glDrawPixels(w,h,GL_RGBA,GL_UNSIGNED_BYTE,fontSurface->pixels);

Works (fontSurface->format->BytesPerPixel returns 4 bytes per pixel)

 

fontSurface=TTF_RenderText_Solid(font, string, col);
glDrawPixels(w,h,GL_RGBA,GL_UNSIGNED_BYTE,fontSurface->pixels);

Does not work (fontSurface->format->BytesPerPixel returns 1 bytes per pixel)

 

I need to convert the surface created by TTF_RenderText_Solid (which seems to be some sort of monohrome surface or something similar) to RGBA surface. I don't know how to make that conversion.


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#4 NewDisplayName   Members   -  Reputation: 359

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Posted 04 January 2013 - 06:14 AM

You shouldn't use GL_RGBA at all in this case. Try this with GL_LUMINANCE.

And, also, you should not convert 8 bpp to 32 bpp (BPP is bits per pixel), as 8 bpp image is a monochrome/grayscale, and it doesn't make any sense to expand it to fit a 32 bpp image without knowing what should be set in which channel.



#5 Acharis   Crossbones+   -  Reputation: 3663

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Posted 05 January 2013 - 09:45 AM

I just want to convert a surface of an unknown/irrelevant type to 32bit RGBA surface. I thought SDL_ConvertSurface() would be able to do it in all cases? Is this correct?

 

Or is there some limitation like it can't be done for palette based surfaces for example?


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#6 darrenc182   Members   -  Reputation: 323

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Posted 06 January 2013 - 11:01 AM

If RGBA is so necessary I don't know why you can't do what I suggested and pack the RGB channels with the same data and set A to 255.  I would ultimately do what NewDisplayName is suggesting, but you seem to be against it.  If you don't mind me asking, what are you trying to achieve?



#7 Acharis   Crossbones+   -  Reputation: 3663

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Posted 16 January 2013 - 05:59 AM

OK, let's try once again :)

I have these two function (from SDL_TTF library):
surface=TTF_RenderText_Blended();
surface=TTF_RenderText_Solid();

Both functions create a surface. The first one creates a nice RGBA surface, no problem here. But the second is creating some other kind of surface. I want them both to create identical kind of surface, so I can render them easily later without changing display modes/data structure, etc. I basicly want to convert the surface of the second function to RGBA surface. It's all on SDL level, no OpenGL involved at that point yet.

 

 

 

If RGBA is so necessary I don't know why you can't do what I suggested and pack the RGB channels with the same data and set A to 255

Because:

1) I don't know how to "pack the RGB channels with the same data" :) That's the whole question here :D

2) Even if I knew how to do it, I can't just set A to 255 since TTF_RenderText_Solid() returns a monohrome picture with same parts fully transparent and some fully opaque (althrough, I could maually change all black to A 0 and all white to A 255, so it's a minor problem).


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#8 Acharis   Crossbones+   -  Reputation: 3663

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Posted 21 January 2013 - 09:17 AM

OK, I got it working.

 

I used SDL_DisplayFormatAlpha() instead, it seems that  SDL_ConvertSurface() is not copying alpha channel.


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